Float valves are well known to cut off the flow of liquid when a desired level of the liquid has been reached in a container. The simplest form of such a valve comprises a float on an end of an arm pivoted to the body of the valve, the other end of the arm pressing on a valve member to close an aperture in an inlet pipe. The float arm is of a length such that the required pressure can be applied to the valve to close against the inlet liquid pressure
In order to obtain further control of the liquid flow and obtain the desired cut off of the flow, servo valves are known, in which a pilot valve controls the operation of a master valve by the application of liquid pressure to assist in the operation of the master valve. The use of a servo valve thus eliminates or reduces the length of the float arm, the servo valve incorporating a diaphragm as the master valve or connected to the master valve.
One example of a diaphragm valve given in AU 659356 in which a sensing or pilot diaphragm has attached thereto a ring magnet. A further magnet is attached to a plunger which is adapted to open and close an aperture in the diaphragm of the master valve.
Various other patent specifications, such as U.S. Pat. Nos. 3,994,313; 4,013,091; 4,977,923; 5,067,516; and 4,027,693; which show servo valves having a diaphragm valve as the master valve which is controlled by a servo valve comprising a solid or mechanical valve controlled by a float valve or other liquid level sensing means.
It is an object of this invention to provide a stop valve in which a smaller force is required to operate the pilot valve, and in turn achieving more precise control of the liquid level in a container such as a cistern or fuel tank or container containing corrosive or other pollutant fluids.
A further object of the invention is to provide a stop valve which is less liable to malfunction due to pollutants in the liquid.
A still further object of the invention is to provide a stop valve having means to reduce or eliminate liquid surge and water hammer in the liquid supply line.